Several studies have shown that 50 to 75% of patients who failed following surgical treatment of a rectal or colorectal cancer failed because of local recurrences. Almost 3/4 of the failures occurred within two years following surgery. The purpose of this ongoing study is to determine if moderate doses of radiation (4600 rads in 5-6 weeks) given electively to high risk areas following surgical resection of locally advanced colorectal cancer (advanced Duke-B or Duke-C) could eradicate subclinical disease. We are therefore evaluating, in this study, the pattern of local recurrence as well as the overall survival of these patients. Thrity-three patients have already entered this study. Twelve had resection for lesions below the peritoneal reflection and 21 for lesions above it. In this short-term followup period, the following results are noted: (1) Almost all patients tolerated treatment well with no short-term sequelae. (2) Local recurrence appears to be lower than that reported in the literature for the same period of time. To determine the effect of wide-field irradiation on the immune system, we are studying cell mediated immunity by the skin reactivity to DNCB and by determining the level of T-lymphocyte by the virus plaque assay. Humoral antibodies are also being determined on all these patients. Alterations in the level of carcino-embryonic antigens (CEA) either because of recurrence or because of bowel irradiation is also being assessed.